1. Field of the Invention
The present invention relates to a photoelectric converting circuit for use in a pickup of an optical recording and reproducing system or for use in a photo sensor provided in various devices such as a tachometer.
2. Description of Background Information
In optical information recording and reproducing systems such as a magneto-optical disc recording system, a light beam is irradiated on an information signal recording track by means of an optical pickup, and a reflected light or a transmitted light thereof is converted to an electric signal, thereby reading an information signal.
An example of such photoelectric converting circuit will be explained with reference to FIG. 1. This figure illustrates an example of the construction of photoelectric converting circuit used in an optical pickup of an optical disc system (not shown). As shown, a light-receiving element 1 made up of a PIN-photo diode and a load resistance R.sub.1 are connected in series with each other, and inserted into a bias power source +B.sub.1. The light-receiving elements 1 is reversely biased through the load resistance R.sub.1, and produces a current responsive to the amount of a light incident upon a light receiving window (not shown) thereof. The light beam is modulated by a signal recorded on the disc, and converted to a radio frequency (referred to as RF hereinafter) current signal by means of the photo receiving element 1. The RF current signal causes a voltage drop which is proportional to the current level thereof across the resistance R.sub.1. A node between the resistance R.sub.1 and the light-receiving element 1 constitutes an output terminal of the photoelectric converting circuit. An output voltage signal appearing at this output terminal is amplified in level by an RF amplifying circuit (not shown), and demodulated subsequently.
FIG. 2 shows an equivalent circuit of the light-receiving element 1, in which the light-receiving element 1 is expressed as a parallel connection circuit of a current source I generating a current i proportional to the intensity of the incident light and a capacitance between the anode and cathode electrodes. The value of the capacitance C is determined by factors such as the reverse bias voltage, and the area of the light-receiving element, and generally is around 20 pF (picofarad).
In order to know the frequency characteristic of the photoelectric converting circuit by using this equivalent circuit, a transfer function F.sub.1 of the photoelectric converting circuit shown in FIG. 1 is determined as follows. Since the capacitance C between the electrodes mentioned before and the resistance R.sub.1 are connected in parallel with respect to the current source I, the transfer function F.sub.1 (s) is expressed by the following expression. ##EQU1##
As shown by the above equation, this circuit forms a first-order lag circuit with a cut-off frequency fc.sub.1 which is equal to 1/2.pi. CR.sub.1. Assume R.sub.1 =10K.OMEGA., C=20 pF, then fc.sub.1 will have a value around 800 KHz (fc.sub.1 .congruent.800 KHz). As a result, frequency components above this frequency are attenuated. If the resistance R.sub.1 is increased above 10K.OMEGA. in order to raise the gain of the circuit, the cut-off frequency will go down to become lower than 800 KHz.
Another example is illustrated in FIG. 3. In the arrangement shown in this figure, the output signal of the light-receiving element 1 is supplied to a negative feedback amplifier made up of an inverting dc (direct current) amplifier 2 and a feedback resistor R.sub.2. An output voltage signal e.sub.2 of the feedback amplifier is fed back to an input thereof at a rate of 100%, so that the voltage level at the input terminal thereof is maintained constant. This voltage at the input terminal serves as the reverse bias voltage of the light-receiving element 1. As the inverting dc amplifier 2, an invertor IC of CMOS (complementary MOS) structure may be used. This negative feedback amplifier constitutes a current-voltage converting circuit, and the output current of the light-receiving element 1 is converted to a voltage by means of the resistance R.sub.2 and outputted subsequently.
The equivalent circuit of this circuit is illustrated in FIG. 4. If the gain of the inverting dc amplifier 2 is denoted by K and the light-receiving element is represented by the current source I and the capacitance C, then the transfer function F.sub.2 (s) thereof is expressed as follows. ##EQU2##
Thus this photoelectric converting circuit forms a first-order lag circuit having a cut-off frequency f.sub.2 which is equal to (K+1)/(2.pi.CR.sub.2). If it is assumed that R.sub.2 =10K.OMEGA., K=10, and C=20 pF, the cut-off frequency fc.sub.2 is approximately at 8.8 MHz. As a matter of practice, since an input capacitance of the inverting dc amplifier 2 is added to the capacitance C of the light receiving element 1, the cut-off frequency becomes to be lower than fc.sub.2.
On the other hand, in magneto-optical disc systems whose developement has been progressing in recent years, a light beam is irradiated on the information recording track of the magneto-optical disc and the read-out of signal is performed by utilizing a small rotation of the plane of polarization in a light reflected by the magneto-optical disc or transmitted through it.
In the case of those magneto-optical disc systems, the RF signal level obtained from the light receiving element such as the element 1 is weak, so that thermal noises generated by the resistance R.sub.1 or R.sub.2 cause a problem in the signal retrieval. Also, it is known that the voltage level of the thermal noise is proportional to the square root of the resistance value. On the other hand, the voltage level of the output signal of the photoelectric converting circuit is proportional to the above resistance value. Therefore, it is desirable to determine the values of the first and second resistances as high as possible in order to raise signal-to-noise ratio (S/N).
However, as mentioned before, if the resistance value is made higher, the cut-off frequency goes down. As a result, it becomes difficult to obtain the required band width. Furthermore, when the RF current signal is weak, there can be another problem of inducing noises.
Specifically, there is a tendency that the higher the resistance value of the load resistance R.sub.1 or R.sub.2 to be connected to the cathrode of the light receiving element 1, the larger the induction noises become. In the case of the photoelectric converting circuits described before, the resistance value of the resistance R.sub.1 is equal to 10k.OMEGA. in the circuit of FIG. 1, and the resistance value of R.sub.2 /(K+1) is approximately 910.OMEGA. in the circuit of FIG. 3. Hence, noises can be induced easily in both examples of the photoelectric converting circuits.